Electropermanent Magnet-based Motors

ABSTRACT

An electropermanent magnet-based motor includes a stator having at least one electropermanent magnet, at least one coil around the electropermanent magnet configured to pass current pulses that affect the magnetization of the magnet, and a rotor that is movable with respect to the stator in response to changes in the magnetization of the electropermanent magnet. A wobble motor has a stator with a centrally-located core from which arms radiate outward, an electropermanent magnet and coil on each arm, and a rotor exterior to the stator such that the rotor can rotate around the stator arms. A rotary motor has a centrally-located rotor that rotates about its axis and a stator exterior to the rotor such that the rotor may rotate within the stator arms, the stator including an anteriorly-located stator core from which stator arms radiate inward toward the rotor, and an electropermanent magnet and coil on each stator arm.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/147,508, filed Jan. 27, 2009, the entire disclosure of which isherein incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with U.S. government support under Grant numberW911NF-08-1-0254, awarded by the Defense Advanced Research ProjectsAgency (DARPA). The government has certain rights in this invention.

FIELD OF THE TECHNOLOGY

The present invention relates to motors and, in particular, to motorsand actuators employing electropermanent magnets.

BACKGROUND

The field of microrobotics seeks to construct mobile robots able tosense, move through, and manipulate their environment with dimensions onthe order of millimeters or smaller. Microrobots have been constructedthat roll, walk, swim, and fly. Microrobots are particularly usefulbecause they can go to places other robots cannot, such as into therubble at a disaster site, into a machine to repair it, or into the bodyfor minimally invasive surgery. A major problem in microrobot design issupplying enough power for long run time. In the fields of microroboticsand programmable matter, there is therefore a need for actuators capableof electromechanical energy conversion at high torque and low speed, andcapable of scaling to small dimensions without loss of efficiency.

The emerging field of programmable matter seeks to build macroscopicobjects with thousands of actuatable degrees of freedom, so that theirshape and function may be changed under software control. Likemicrorobots, programmable matter will also require efficient small-scaleactuators.

A wide variety of micromotors and microactuators have been constructedthat have their largest dimension measured in millimeters. Theseactuators have used a variety of operational principles, includingmagnetic, electrostatic, piezoelectric, and electrothermal. Because itis difficult and expensive to fabricate efficient speed-reducing powertransmissions at microscale, an actuator that directly produces the hightorques at low speeds efficiently would be desirable for robotics andprogrammable matter. Such an actuator could be advantageously be appliedeven with a speed-reducing (or speed-increasing) power transmission, butthe transmission might not require as many stages, have as large a speedratio, or as large a maximum speed as it would otherwise.

Magnetic motors dominate electrical to mechanical energy conversion atmacroscale, powering a wide range of devices from industrial machinetools to household appliances to children's toys. These motors areavailable in a wide range of types (e.g. induction, servo, stepper) andconfigurations (e.g. rotary, linear) depending on the requirements ofthe application. Magnetic motors have two halves, a rotor and a stator.Electrical energy provided from an external source is used to produce achanging magnetic field at the interface between the rotor and stator,which propels the rotor and produces useful mechanical work.

Since the construction of the first electric motor by Michael Faraday in1821, permanent magnets have been used in the construction of motors. Insmall electric motors (having a capacity less than one horsepower)permanent magnets are commonly used in order to improve efficiency andtorque, and to simplify construction and drive. For example, in thebrush-commutated permanent-magnet DC motor, a periodically-reversingcurrent through coils on the rotor interacts with the magnetic field ofpermanent magnets on the stator to produce torque. A key designcriterion for almost all permanent magnet motors is that thedemagnetizing field inside the permanent magnets must not exceed a thedemagnetizing threshold. Otherwise, the magnetic field of the magnetswill be reduced by operation, and the motor will not function as well.Conventional permanent magnet motors are carefully designed to avoiddemagnetization of their permanent magnets.

A notable exception is the hysteresis motor (such as, for example, thatdisclosed in U.S. Pat. No. 3,610,978) which works by continuouslycycling a piece of magnetic material around its hysteresis loop, andgenerating continuous torque due to the time lag between field and fluxwhile changing the magnetization of the material. The hysteresis motoris notable and useful for its ability to produce constant torqueindependent of speed. However, the hysteresis motor requires thecontinuous input of electric power to produce continuous torque, even atzero speed, and so has low efficiency at low speeds.

Electric motors suffer from a number of loss mechanisms that act toreduce their power efficiency. At high speeds, mechanical losses due tofriction, and magnetic losses due to cyclic magnetization anddemagnetization of the flux-carrying members dominate. Thus,flux-carrying materials (e.g. iron) for magnetic motors are typicallyselected to have the lowest possible coercivity and thus the lowestpossible magnetic losses. At low speeds, loss due to resistive heatingof the coils dominates. At the limit of zero speed, when the motor isstalled, 100% of the electrical energy input goes to resistive heatingof the windings and the motor operates at zero percent efficiency.

SUMMARY

Motors and actuators according to the present invention have at leasttwo sections (e.g. rotor and stator) that move relative to one another.At least one of these sections includes one or more electro-permanentmagnets. Passing current through (the coil of) these electropermanentmagnets changes the magnetization of the materials inside, storingenergy in the magnetic materials, changing the force they exert on theother member, and causing relative movement. After the current isremoved, the motor continues to exert force and do work for a period oftime, as stored magnetic energy in the magnets is converted tomechanical work. Applying sequential current pulses to multipleelectropermanent magnets results in continuous motion. Also, sensing therelative positions of the sections and controlling the timing,magnitude, or shape of the applied pulses allows for precise control ofposition or speed. The relative position of the sections may be sensedby external means, or may be sensed by measuring the voltage or currentin the coil. A motor according to the invention can produce torqueefficiently when operated at low speeds, or equivalently, when the motoris constructed at small dimensions, a large rotational speed (inrevolutions per minute) will still result in a low linear speed betweenrotor and stator. A motor according to the invention does not requireelectrical power in stall, so that it can operate with greater than zeroefficiency as the speed approaches zero.

In one aspect, the invention is an electropermanent magnet-based motorthat includes a stator having at least one electropermanent magnet, atleast one coil that is wound around each electropermanent magnet andconfigured to pass current pulses that affect the magnetization of themagnet, and a rotor that is movable with respect to the stator inresponse to changes in the magnetization of the electropermanentmagnets. In a preferred embodiment, the electropermanent magnets eachcomprise two independently-controllable magnets made of differentlymagnetizable materials.

In one aspect, the invention is an electropermanent magnet-based motorhaving a wobble configuration, including a stator having acentrally-located stator core from which stator arms radiate outward, anelectropermanent magnet that is integral to each stator arm, and a coilthat is located around the electropermanent magnet on each stator armand a rotor positioned exterior to the stator in such a configurationthat the rotor can rotate around the stator arms. In some embodiments,the stator core and stator arms may be formed from a single piece ofmaterial such as, but not limited to, iron. In a preferred embodiment,there are two independently-controllable magnets attached to each statorarm, each made from a material having a different coercivity. In apreferred embodiment, one has a very high coercivity and one has asubstantially lower coercivity, but still substantially higher than thecoercivity of the flux-guiding members. The end of each stator arm andthe interior of the rotor may have integrated gear teeth designed tomesh with each other. Alternatively, or in addition, there may be ahigh-friction coating the rotor and/or stator surfaces. One embodimentincludes dual coaxial gear wheels with gear wheel teeth located aboveand below the plane occupied by the stator and the rotor is a ringsandwiched between two rotor gears having rotor teeth designed to matewith the gear wheel teeth of the stator gear wheels.

In another aspect, the invention is an electropermanent magnet-basedmotor having a rotary configuration, including a centrally-located rotorconfigured to rotate about its axis, optionally about a shaft, and astator that is exterior to the rotor, the rotor being located within thestator in such a way that the rotor may rotate within the stator arms,the stator including an anteriorly-located stator core from which statorarms radiate inward toward the rotor, an electropermanent magnet that isintegral to each stator arm, and a coil that is located around theelectropermanent magnet on each stator arm. In some embodiments, thestator core and stator arms may be formed from a single piece ofmaterial such as, but not limited to, iron. In a preferred embodiment,there are two independently-controllable magnets attached to each statorarm, each made from a material having a different coercivity. In apreferred embodiment, one has a very high coercivity and one has asubstantially lower coercivity, but still substantially higher than thecoercivity of the flux-guiding members. The end of each stator arm andthe exterior of the rotor may have integrated gear teeth designed tomesh with each other. Alternatively, or in addition, there may be ahigh-friction coating the rotor and/or stator surfaces.

The invention advantageously utilizes electro-permanent magnets inmotors and actuators. Motors and actuators using electro-permanentmagnets are useful over the present art in motor design and constructiondue to their ability to achieve high efficiency when run at low speedsand/or when fabricated with small dimensions. Because current flowsthrough the windings for only a small fraction of the time that themotor is producing torque, and that fraction is lower when the motorturns more slowly, losses due to resistive heating of the windings whenthe motor is run at low speeds are greatly reduced. With a motorconstructed according to the present invention, there is a relativelyconstant resistive and hysteresis energy loss per revolution,independent of speed, in contrast to a conventional permanent-magnetmotor, where the energy is continuously dissipated due to resistivepower loss, even if the motor is stalled. The technology of the presentinvention is particularly suitable for micromotors and microactuatorsfor use in the fields of microrobotics and programmable matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, advantages and novel features of the invention willbecome more apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawingswherein:

FIGS. 1A and 1B depict an exemplary embodiment of an electropermanentstepper motor, wobble configuration, before and after the installationof coils, respectively, according to one aspect of the presentinvention;

FIGS. 2A and 2B depict a plan view of an exemplary embodiment of anelectropermanent stepper motor, wobble configuration, before and afterthe installation of coils, respectively, according to one aspect of thepresent invention;

FIG. 3 depicts a drive waveform for an exemplary embodiment of anelectropermanent stepper motor, wobble configuration, according to oneaspect of the present invention;

FIG. 4 depicts a prototype embodiment of an electropermanent steppermotor, wobble configuration, according to one aspect of the presentinvention, with associated drive circuitry, and a quarter shown forscale;

FIG. 5 is a diagram of the magnetic field diagram inside an exemplarystepper motor according to one aspect of the present invention, showingcirculating flux in the inactive arms and flow of flux through the twoactive arms in a large loop;

FIGS. 6A-C depict an exemplary embodiment of an electropermanent steppermotor, wobble configuration, with a monolithic iron stator flux-guide,before the installation of the rotor and coils (FIG. 6A), afterinstallation of the rotor (FIG. 6B), and after installation of both therotor and coils (FIG. 6C), according to one aspect of the presentinvention;

FIGS. 7A and 7B depict an exemplary embodiment of an electropermanentstepper motor, wobble configuration, with integrated gear teeth on rotorand stator, according to one aspect of the present invention;

FIGS. 8A-D depict an exemplary embodiment of an electropermanent steppermotor, wobble configuration, with coaxial gearing, according to oneaspect of the present invention;

FIGS. 9A and 9B depict an exemplary embodiment of an electropermanentstepper motor, rotary configuration, before and after the installationof coils, respectively, according to one aspect of the presentinvention;

FIG. 10 is a plan view of an exemplary embodiment of an electropermanentstepper motor, rotary configuration, before the installation of coils,respectively, according to one aspect of the present invention;

FIGS. 11A and 11B depict an exemplary embodiment of an electropermanentlinear actuator, according to one aspect of the present invention; and

FIG. 12 depicts an exemplary embodiment of an electropermanent inchwormmotor, according to one aspect of the present invention.

DETAILED DESCRIPTION

The present invention employs electro-permanent magnets in motors andactuators. Motors/actuators according to the present invention have atleast two sections, rotor and stator, that move relative to one another.At least one of these sections includes one or more electro-permanentmagnets. Passing current through the coil of the electropermanentmagnets changes the magnetization of the materials inside, changing theforce they exert on the other member, causing relative movement.Applying sequential current pulses to multiple electropermanent magnetsresults in continuous motion. Also, sensing the relative positions ofthe sections and controlling the timing, magnitude, or shape of theapplied pulses allows for precise control of position or speed. Therelative position of the sections may be sensed by external means, ormay be sensed by measuring the voltage or current in the coil.

As used herein, the following terms expressly include, but are not to belimited to:

“Electropermanent magnets” means magnetic assemblies whose holding forcecan be substantially modified by electrical pulses. An electropermanentmagnet is a device undergoes a substantial change in its magnetizationupon stimulus with a current pulse. After the current is returned tozero or a low level, the magnetization remains substantially changed. Anelectro-permanent magnet can receive energy in a short current pulse,and after the pulse is completed, continue to apply a magnetic force onanother member through an air gap for a much longer time than the lengthof the pulse. Electropermanent magnets are further described in, forexample, U.S. Pat. Nos. 4,075,589, 6,002,317, and 6,229,422.

“Rotor” means the rotating member of an electrical machine or device,such as the rotating armature of a motor or generator.

“Stator” means the portion of a rotating machine that contains thestationary parts of the magnetic circuit and, sometimes, theirassociated windings.

Because electropermanent magnets pull with significant force even atlarge distances and consume low power at small dimensions, motors andactuators employing electropermanent magnets according to the presentinvention are extremely efficient, consuming low power at smalldimensions and slow speeds. A number of different embodiments have beendesigned and implemented, including an electro-permanent stepper motorwith a wobble configuration, an electro-permanent stepper motor with arotary configuration, electro-permanent motors with a monolithic statorcore, electro-permanent motors with integrated gear teeth,electro-permanent motors with coaxial contact wheels, and anelectro-permanent linear actuator. In one embodiment, anelectro-permanent stepper motor is used to actuate the rotary joints ofa programmable matter system.

Electropermanent Stepper Motor with Wobble Configuration. In oneexemplary embodiment, called the “Electropermanent Wobble Stepper Motor”there are two sections, a rotor and a stator. The stator is in thecenter, and the rotor revolves around it. In a preferred embodiment,most parts of the rotor and stator are made of iron, selected for itslow coercivity and high saturation flux density. The outer profile ofthe rotor is generally circular in shape with a given diameter, and theinner profile of the rotor is circular in shape, but with a slightlysmaller diameter. Each arm of the rotor contains an electropermanentmagnet. These electropermanent magnets consist of two permanent magnetmaterials placed side-by-side. In a preferred embodiment, one materialis Nd-Fe-B (neodymium-iron-boron) permanent magnet alloy and the othermaterial is Alnico 5 alloy (aluminum-iron-nickel-cobalt-copper). A coilof copper wire is wrapped around each arm of the stator. The coils onopposing arms are connected together, forming a four-wire device withtwo electrical phases.

FIGS. 1A and 1B depict an exemplary embodiment of an electropermanentstepper motor, wobble configuration, before and after the installationof coils, respectively, according to one aspect of the presentinvention. In the exemplary embodiment of FIGS. 1A and 1B, rotor 105 isoutside stator 110, which comprises stator core 115, optional statormounting hole 120, coils 125, and stator arms 130, which comprise Alnicomagnets 135, NdFeB magnets 140, and stator arm ends 145. FIGS. 2A and 2Bdepict a plan view of the exemplary embodiment of FIGS. 1A and 1B,respectively.

An electropermanent wobble stepper motor according to the presentinvention operates as follows: In the initial condition, two adjacentmagnets are on and the other two are off. This causes the rotor toadhere to the stator at the ends of the two stator arms for which themagnets are on. A pulse of current through one the electrical phases, inthe proper direction, simultaneously turns off the magnet on one of itsarms and turns on the magnet on its opposing arm. Now the rotor pivotsaround the magnet on the perpendicular arm, which stays on continuously,moving away from the magnet that switched off and toward the magnet thatswitched on. This results in a rotation and translation of the rotorrelative to the stator. By energizing the electrical phases as shown inthe figure, rotation can proceed in either direction.

A prototype embodiment of an electropermanent stepper motor with thisconfiguration was fabricated in the lab. The iron portions of the rotorand stator were made from ¼ inch thick iron plate, grade ASTM-A848,purchased from Scientific Alloys, Inc (Westerly, R.I.). The permanentmagnets were ⅛″ diameter and ⅛″ long. The Nd-Fe-B magnets were purchasedfrom Amazing Magnets, Inc. (Irvine, Calif.), part number R125A. TheAlnico magnets were purchased from McMaster-Carr Industrial Supply Co.(Robinsville, N.J.), part number 5852K11. As purchased, the alnicomagnets were longer than the needed length. The magnets were cut tolength using a diamond saw, purchased from MTI Corporation (Richmond,Calif.), model number EC400. A section of the iron plate was cut out andsurfaced flat and parallel to ¼″ using a vertical milling machine (HAASSuper Mini-Mill, HAAS Automation Inc., Oxnard, Calif.). Then, anabrasive water jet cutter (OMAX JetMachining Center, Model 2652, OMAXCorporation, Kent, Wash.) was used to cut the online of the center ofthe stator. The stator was secured in the mill with a screw through thecenter hole and wax-gluing. The vertical mill was then used to surfacethe four magnet-mating ends, making them flat, smooth, and square. Thecenter of the stator was removed and cleaned in heated isopropanol,heated to 50 C, until no more wax was visible.

The magnets were glued to the center of the stator, in the configurationshown in FIGS. 1A-B and 2A-B, using cyanoacrylate adhesive (Loctite 409Gel Adhesive, Henkel Corporation, Dusseldorf, Germany). Each arm of thestator has two Nd-Fe-B magnets and two Alnico magnets. The two Nd-Fe-Bmagnets on each arm have the same polarity. The Nd-Fe-B magnets onadjacent faces have opposing polarity. For example, if the magnets onone arm are mounted north-out, the magnets on the adjacent arms shouldbe north-in. The mounting polarity of the Alnico magnets duringconstruction is not critical, since they will have their magnetizationchanged during operation, but it is easier to assemble the device ifthey are mounted with opposite polarity to the Nd-Fe-B magnets on theirarm.

The ends of the stator arms were cut out, tabbed together, on theabrasive waterjet, and the magnet-mating side was finished on thevertical milling machine. The tabs holding the pieces together were cutusing a hacksaw, and the remnants of the tabs were filed down using ametal file. The stator ends were cleaned in heated isopropanol at 50 C.The stator ends were then glued onto the magnets using Loctite 409adhesive, and 24 hours were allowed to pass in order for the glue tofully set. The stator was fastened for machining on the vertical mill,again using the screw and wax-gluing method for mounting. Using a largenumber of gentle passes with an end mill, a circular profile was cut onthe outside of the stator.

Next, 80 turns of 33AWG bondable magnet wire (MWS Wire industries,Westlake Village, Calif.) were wound around each arm. Opposing magnetsare electrically connected and wound in the same direction (e.g.counterclockwise) so the device has four leads and two electricalphases. The wires were secured in place at the four corners of thestator with a few drops of the cyanoacrylate adhesive. Then the statorwas machined. The profile of the stator was cut out from the ¼″ ironplate, keeping the inner circle slightly undersized, and the innercircle was finished with a 13/16″ reamer.

To drive the motor, two open-source motor controller boards (Robot PowerInc, S. Roy Wash.) were used, one for each electrical phase, bothcontrolled by a single ATMEL AVR microcontroller (Atmel Corporation, SanJose, Calif.). The microcontroller was programmed to drive the twophases with the pulse sequence shown in FIG. 3, using 100 microsecondlong pulses, and spacing the pulses 10 milliseconds apart. The powersupply voltage was 24V. Two 1000uF decoupling capacitors in parallel onthe power rails provided the needed charge to supply the required 5-10Ainstantaneous current to drive the motor.

FIG. 4 depicts another prototype embodiment of an electropermanentstepper motor, wobble configuration, according to one aspect of thepresent invention. Shown in FIG. 4 are stepper motor 410, includingrotor 420, stator core 430, and coils 440, associated drive circuitry450, and a quarter 460 shown for scale.

FIG. 5 is a diagram of the magnetic fields inside an exemplary steppermotor according to one aspect of the present invention, showingcirculating flux in the inactive arms (in a small loop, between the twomagnets in the arm) and flow of flux through the two active arms (in alarge loop, through both arms). Test results for the pictured stepperwere 50 gram-mm torque and 1.2 Joules/Revolution energy consumption.

It will be clear to a person skilled in the art of the invention thatmany wiring and drive variants are possible. For example, one wire fromeach electrical phase of the motor may be connected together to form acommon lead, and then the motor would become a three-wire device. Thiscould be advantageous, because it would reduce the number of wires toconnect the device to the drive circuit, and reduce the number ofswitches (e.g. transistors) required of the drive circuit, while stillallowing the drive waveforms shown in the figure. In a motor with morethan two electrical phases, similar common connections of leads would bepossible. Alternatively, in another wiring variant, each side of each ofthe coils of the motor could be connected independently, making themotor, with four arms, into an eight-wire device. In yet anotheralternative, one lead of each coil could be connected to a common, andthen the other wire from each coil could be connected to a common,making the four arm motor a five-wire device. These wiring arrangementswould allow opposite coils to be activated independently, which could beadvantageous because shifting the pulse that turns on and off oppositecoils somewhat in time could result in energy savings.

It will further be clear to a person skilled in the art that differentnumbers of stator arms are also possible. In different configurations,there are constraints on the number of stator arms. For example, usingthe Nd-Fe-B/Alnico electropermanent magnets that are only designed to beswitched to one polarity or off, and designing to keep magnetic fluxinside the iron as much as possible, there is a need for the polaritiesof the magnets to alternate around the outside, and this limits thenumber of arms to even numbers. However, using electropermanent magnetsthat can be switched to either polarity, or in systems where continuousrotation is not required, or using a mix of electropermanent magnets andelectromagnets, or with a variety of other variations to the design, andodd number of poles are possible and may be advantageous in certainapplications.

It will also be clear to a person skilled in the art that thefunctionality of the rotor and stator may be modified. For example, thepart called the rotor could be stationary, and the part called thestator could rotate. In the embodiments described, the coils are placedon the stationary part for ease of wiring, making these embodimentsbrushless motors. However, it is possible to construct a variant of thisdesign using a mechanical commutation to sequence the pulses, and thenit would be advantageous for the portion of the motor containing thecoils to move. Similarly, if the drive circuit was on the rotating part,then it would be advantageous for the coils to be on the rotating part.

It will further be clear to a person skilled in the art that the partwith the coils may be external, and the other part could be internal(e.g. a rotating round shaft surrounded by magnets). An embodiment withthis configuration is discussed later. While the external rotorconfiguration was employed in the preferred embodiment for its increasedtorque, machine configuration requirements, manufacturability, or avariety of other considerations might make the magnets-externalconfiguration preferable.

It will similarly be clear to a person skilled in the art that materialsother than the iron, Nd-Fe-B alloy, Alnico 5 alloy, and copper used inthe disclosed embodiments could be advantageously employed in thepresent invention. The iron was chosen for its high permeability, lowcoercivity, and high saturation flux density. The Nd-Fe-B was chosen forits high remnant flux density and high coercivity. The Alnico 5 waschosen for its similar remnant flux density to the Nd-Fe-B and itscoercivity between that of the iron and that of the Nd-Fe-B.

It will additionally be clear to a person skilled in the art that themotor described above, with its eccentric rotation pattern, could becoupled to a shaft in more-or-less pure rotation (e.g. a shaft fixed bya bearing) using a variety of misalignment coupling techniques, such as,but not limited to, a bushed pin type coupling, Universal coupling,Oldham coupling, Bellows coupling, Spider coupling, Thompson coupling,Resilient Coupling, or Disc coupling. It will also be clear to a personskilled in the art that several of these motors could be connected to ashaft with couplers, with at least some of the motors out of phase toone another, to realize what is known in the electric motor art as amultistack stepper motor.

In addition, it will be clear to one of skill in the art that some partsof the rotor or stator serving a primarily structural function might bemade of a nonmagnetic material, rather than a magnetic material such asiron. This might be preferable to reduce weight or to increasemechanical strength. The rotor and stator may be made of a laminatedstack of layers of magnetic material interspersed with layers ofnon-conductive material, to reduce eddy current losses.

In another variant of the invention, the electropermanent magnets on thearms could use two bars of the same, or similar, material in parallel,each surrounded by a separate coil. This magnet could then be set toeither magnetic polarity, or off. This could be desirable because itwould allow a motor with an odd number of arms, for example, three. Inanother alternative, the electropermanent magnets on the arms could be asingle block of one material, wound with a coil. In this variant, themagnet would be turned on by a large energy pulse, and then turned offby an alternating sequence of opposite polarity pulses with decreasingenergy. This could be desirable because it involves a smaller number ofmaterials.

In another variant of the invention, one or more of the magnets isdriven partially, not all the way to saturation. This could be done, forexample, by a high-energy magnetizing current pulse in one direction,followed by a shorter, lower-energy demagnetizing pulse in the otherdirection. This would allow for micro-stepping, for finer positioncontrol than possible with discrete steps. In another variant of theinvention, a heater is used to heat one or more of the permanent magnetmaterials before or during the current pulse, reducing the energy of thepulse required to switch its magnetization.

In another variant of the invention, rather than using two hard magneticmaterials of differing coercivity in parallel in the magnetic circuit, athick and thin piece of the same material could be placed in parallel inthe magnetic circuit. In this variant, the thicknesses would be selectedso that the magnetomotive force from the current pulses through the coilwould be sufficient to flip the magnetization in the thin piece ofmaterial, but not in the thick piece.

In one embodiment of the invention, the mechanical interface isseparated from the magnetic interface by sandwiching the existing statorbetween two slightly-larger-diameter plates, so the mechanical contactis made with the plates rather than with the magnetic components. Thisis advantageous because then the contact components can be made of ahard, wear-resistant, non-corroding, non-magnetic material such as, butnot limited to, stainless steel and because assembly and manufacturingare simplified.

The magnet-containing arms of any of the motors described herein may beconstructed with a monolithic flux-guiding structure. FIGS. 6A-C depictan exemplary embodiment of an electropermanent wobble stepper motor thatis similar to the embodiment of FIGS. 1A-B and 2A-B, but with theflux-guiding portion of the stator being made from a single piece ofmaterial. In the embodiment of FIGS. 6A-C, monolithic iron statorflux-guide 610 is formed from a single iron piece, integrating thestator core and stator arm tips. Inserted into stator core 610 areAlnico magnets 620 and Nd-Fe-B magnets 630. As shown, stator core 610also has optional mounting hole 640. Rotor 650 rotates around stator610. Coils 660 are wrapped around or inserted over the arms of stator610 in order to permit switching of the magnetization of theelectropermanent magnet on each arm. Note that after insertion ofmagnets 620, 630, a thin metal tether 670 runs from the core of stator610 to the tip of each arm. It will be clear to one of skill in the artthat this pictured embodiment is exemplary only, and that any of thevariations discussed in conjunction with the embodiment of FIGS. 1A-Band 2A-B would also be suitable for use in an embodiment with amonolithic stator.

Using this design, stator fabrication and motor assembly is simplified,since the flux-guiding portion can be cut from a single piece ofmaterial (for example, but not limited to, iron) using a low-costtwo-dimensional manufacturing process, and then the magnets and coilsare assembled onto that piece. In the design shown in FIGS. 6A-C, therewill be flux leakage in the thin iron tether 670 connecting the statorarms to the stator core, because the thin metal arm conducts back someof the magnetic flux, and this flux leakage will tend to reduce theperformance of the motor. However, this design may still be preferablefor some applications, particularly because of its advantages inmanufacturability.

Electropermanent Stepper Motor with Integrated Gear Teeth. In anotherembodiment of the invention, the rotor and the stator have protrudingteeth. Preferably, the pitch of these teeth is set so that the motoradvances by an integer number of teeth per step. This is preferablebecause it allows the motor to resist larger opposing torques withoutslipping. Adding gear teeth at the rotor/stator interface reduces thereliance on friction for operation, improving torque and reliability.

FIGS. 7A and 7B depict an exemplary embodiment of an electropermanentstepper motor, wobble configuration, with integrated gear teeth on therotor and stator. In the exemplary embodiment of FIGS. 7A and 7B,one-iron-piece stator 710 has integrated stator gear teeth 720 on theend of each arm. Each arm of stator 710 also has Nd-Fe-B magnet 730 andAlnico magnet 740 inserted into slots in the stator flux-guide 710,which has optional mounting hole 750. The coils in this embodiment arenot shown in FIGS. 7A-B so that magnets 730, 740 are visible, but arelocated as in the exemplary embodiment of FIGS. 1A-B and 2A-B. Stator710 meshes with rotor 760, which was constructed from iron in theprototype embodiment, at rotor gear teeth 770. Gear teeth 720, 770 forstator 710 and rotor 760 are preferably fabricated onto their contactingsurfaces.

When the properties of a prototype of the embodiment of FIGS. 6A-C weremeasured, it was found that the motor torque is generally limited byslip between the rotor and stator. One approach to reduction of slip isthe use of a high-friction coating such as silicone rubber or sand onthe rotor or stator surfaces. Another approach is the fabrication ofgear teeth onto the rotor and stator, as in the embodiment of FIGS.7A-B. The gear teeth increase the torque needed to cause the motor toslip, by increasing the pressure angle between the rotor and stator. Itwill be clear to one of skill in the art of the invention that acombination of these two approaches, gear teeth with a high-frictioncoating, is also possible and may be advantageously employed in certainapplications. The teeth should be constructed so that the rotor andstator teeth have the same module, in order to allow them to mesh. Theuse of teeth on wobble motors in order to increase running torque isfurther described in, for example, Suzumori, K. Hori, K., “Microelectrostatic wobble motor with toothed electrodes”, Micro ElectroMechanical Systems, 1997. MEMS '97, Proceedings, IEEE., Tenth AnnualInternational Workshop on, pp. 227-232, Nagoya, Japan.

Electropermanent Wobble Motor with Coaxial Gearing. In the design of amotor with integrated gear teeth, because the tooth tips serve both amechanical and a magnetic function, there are trade-offs selection ofthe material and design of the shape of the teeth; where one design orselection would be preferable from a magnetic perspective, anotherdesign or selection may be preferable from a mechanical perspective. Forexample, while mechanically it would be preferable to construct the gearteeth from a hard, durable, and non-corroding material, such as, but notlimited to, sapphire-coated stainless steel, these materials havegreatly inferior magnetic properties when compared to specializedmagnetic materials such as annealed pure iron, iron silicon alloys, andiron-cobalt-vanadium alloys (which in turn have inferior mechanicalproperties). In finite-element studies of the field patterns from gearedteeth, it has been found that the direction of the magnetic field isinfluenced by the tooth shape. Therefore, the optimal magnetic shape fortooth tips may be different than the optimal mechanical shape.

In order to address both of the foregoing issues, an electropermanentwobble motor can be constructed with co-axial contact wheels, e.g.toothed gear wheels, attached above and below the stator. The wheels mayhave gear teeth to transmit torque, as shown in FIGS. 8A-D, or maysimply be circular rollers. The contact wheels are designed to protrudefarther than the magnetic components, so that that contact wheels comeinto contact with one another, while the magnetic components act on eachother without touching. Therefore, the contact wheels will provide themechanical function, the magnetic components will provide the magneticfunction, and the design of each can then be optimized separately forbest performance. For example, the contact wheels may be constructed ofstainless steel, and the arm tips may be constructed of annealedmagnetic-grade iron.

FIGS. 8A-D depict an exemplary embodiment of an electropermanent steppermotor, wobble configuration, with dual coaxial gear wheels. FIG. 8Ashows the stator assembly and lower gear wheel, with top wheel havingbeen removed for clarity. FIG. 8B depicts the corresponding rotor. FIG.8C depicts the stator and gear wheel assembly before installation of therotor, and FIG. 8D depicts the full assembly (rotor, stator, and bothcoaxial gear wheels). In the exemplary embodiment of FIGS. 8A-D, stator805 has four Alnico magnets 810, four Nd-Fe-B magnets 815, monolithicstator flux-guiding component 820 with semi-cylindrical faces, andoptional mounting hole 825. Coils 830 provide for the magnetization ofelectro-permanent magnets 810, 815 to be switched. The magneticcomponents are sandwiched between two stainless steel stator gear-wheels840, 845, with composite stub-involute/arc gear teeth 850, 855 on thesurface of each. Also shown are optional dowel-pin holes 860, which maybe advantageously employed to simplify alignment between top 840 andbottom wheels 845. Rotor 865 comprises iron ring 870, sandwiched betweentwo rotor gears 875, 880, having teeth 885 designed to mate with teeth850, 855 of stator gear wheels 840, 845. The mating gears have the samemodule (ratio of number of teeth to diameter), in order to allow meshing[Suzumori, K. Hori, K., Micro electrostatic wobble motor with toothedelectrodes, Micro Electro Mechanical Systems, 1997. MEMS '97,Proceedings, IEEE., Tenth Annual International Workshop on, pp. 227-232,Nagoya, Japan]. In the embodiment shown, rotor gears 875, 880 have onemore tooth each than stator gears 840, 845, so that the stator rotates asmall amount each time the magnetization of the stator arms is switched,in a manner similar to the mode of operation described for the exemplaryembodiment of FIGS. 1A-B and 2A-B.

One further advantage of this design is that it is possible to designthe wheels so that there is a continuously moving, single line ofcontact between the inner and outer contact wheel (for example,according to the procedure in Suzumori, K., Hori, K., Microelectrostatic wobble motor with toothed electrodes, Micro ElectroMechanical Systems, 1997. MEMS '97, Proceedings, IEEE., Tenth AnnualInternational Workshop on, pp. 227-232, Nagoya, Japan). This simplifieskinematic analysis of the motor. For balance and stability, it isgenerally advantageous to have two contact wheels, one on the top andone on the bottom of the magnetic components, but this should not beseen as limiting the scope of the present invention and otherconfigurations may be advantageous for certain applications.

Electropermanent Stepper Motor with Rotary Configuration. In anotherpreferred embodiment of the invention, there is an inner rotor and outerstator. The rotor and stator are concentric. The rotor is fixed with abearing so that it is free to rotate relative to the stator, but not tomove substantially in any other direction. In one preferredimplementation, both the rotor and stator are made substantially of ironand the stator has six arms. Each arm contains an electropermanentmagnet. The electropermanent magnets consist of an Nd-Fe-B magnet and anAlnico magnet, in parallel. Each arm has a coil of copper wire wrappedaround it. Opposite coils are connected together into three electricalphases, yielding a six-wire device. Both the rotor and stator havesalient teeth placed on the same pitch. The teeth on each pair ofopposite arms of the stator are out of phase with each other, in themanner of a standard toothed stepper motor. Magnets on opposite arms arepolarized such that, when opposing magnets are turned on, magnetic fluxmay flow through both opposing magnets.

FIGS. 9A and 9B depict an exemplary embodiment of an electropermanentstepper motor, rotary configuration, before and after the installationof coils, respectively, and FIG. 10 is a plan view of an exemplaryembodiment of an electropermanent stepper motor, rotary configuration,before the installation of coils, according to this aspect of thepresent invention. In the embodiment depicted in FIGS. 9A-B and 10,stator 910 is exterior to rotor 920 and comprises Alnico magnets 930,NdFeB magnets 940, stator arm ends 950, optional stator mounting holes960, and coils 970. Rotor 920 turns on shaft 980. Rotor 920 and statorarm ends 950 may have optional teeth 990, 995, respectively.

Operation of a motor according to this embodiment is as follows:Starting with all magnets off, the rotor is free to rotate. A currentpulse through one electrical phase in the proper direction turns on twoopposing magnets. The rotor rotates to cause the poles of the rotor andof those opposing stator arms to come into alignment, in a position ofminimum air-gap reluctance. The motor will exert torque to hold therotor at this position without further power input. When the next stepis desired, an opposite current pulse disengages the first arm, and acurrent pulse through the second electrical phase activates the secondarm, pulling the rotor into the next position.

In an alternative embodiment, turning on the magnets connected to morethan one electrical phase at a time results in the rotor coming to restat an intermediate position, a behavior known to those skilled in theart of electric motors as micro-stepping. In another alternative,turning on the magnets connected to one or more electrical phases withdifferent degrees of magnetic field, for example by applying a largeforward current pulse and then a smaller reverse current pulse, allowsthe motor to apply a tunable amount of torque, and also allows the motorto be set to come to rest at arbitrary intermediate positions betweensteps. It will further be clear to one of skill in the art that thispictured embodiment is exemplary only, and that many of the variationsdiscussed in conjunction with the embodiment of FIGS. 1A-B and 2A-Bwould also be suitable for use in a rotary configuration embodimentaccording to the present invention.

Electropermanent Linear Actuator. In one preferred application of theinvention, electropermanent magnets are used in a linear actuator. Inthis embodiment, one member, the forcer, is constrained by a linearbearing to be free to move in one linear direction but not to movesubstantially in any other direction. A spring connects the forcer tothe stator. An electropermanent magnet applies a force to pull theforcer to one position when the magnet is on, and another position whenthe magnet is off.

FIGS. 11A and 11B depict exemplary embodiments of an electropermanentlinear actuator, according to one aspect of the present invention.Depicted in FIGS. 11A and 11B are forcer 1105, spring 1110, stator 1115,1118, actuation shaft 1120, linear bearing 1125, Alnico magnet 1130,neodymium magnet 1140, magnetic field air gaps 1150, 1155, 1160, 1165and coils 1170, 1175.

One application for such an actuator is a fluidic value (for example,but not limited to, for switching a flow of air or water). A currentpulse in one direction opens the valve. A current pulse in the otherdirection closes the valve. No electrical power is required to keep thevalue in the same state. Other applications, such as a latchingelectrical relay, or any application normally served by a linearactuator, are possible.

As will be apparent to one skilled in the art of mechanical design, thespring does not need to be a discrete element, but may alternatively bea flexure, so that the forcer and stator are one mechanical part butdesigned to have a low stiffness in one direction so that theelectropermanent magnet can move them relative to one another. In analternative embodiment, there is no spring or flexure, and twoelectropermanent magnets are used. In one position, one is on and theother is off. In the other position, the activation of the magnets isreversed. In yet another alternative embodiment, multiple magnets areused, and one or more is switched on to move the forcer to differentpositions by applying different amounts of force. It will further beclear to one of skill in the art that these embodiments are exemplaryonly, and that many of the variations discussed in conjunction with themotor of FIGS. 1A-B and 2A-B would also be suitable for use in anelectropermanent linear actuator according to the present invention.

In one alternative embodiment, the electropermanent magnet or magnetsare activated with a controllable force level to move the actuator to aprecise position. This may be used to make, for example, a proportionalflow-rate value. In another alternative embodiment, more than one axismovement is allowed, and controllable using multiple electro-permanentmagnets. For example, a hexapod six-degree-of-freedom flexural positingstage may be used with six electro-permanent magnets, whose force iscontrollable, to allow positioning a stage with six degrees of freedom.This may be useful, for example, as part of a micro-machining tool, oras the stage of a microscope.

Electropermanent Inchworm Motor. In another preferred embodiment,electropermanent magnets are used to build an inchworm motor, capable ofcontrolled movement along a surface made of iron or other magneticmaterial. An inchworm motor can be used, for example, for programmablepositioning of an optical component on an iron plate. For a linearinchworm motor, two electropermanent magnets are used as clamps to holdthe motor to the iron table. A linear actuator, such as, for example,but not limited to, the one described previously, connects the twohalves of the motor together. FIG. 12 depicts an exemplary embodiment ofan electropermanent inchworm motor, according to one aspect of thepresent invention. As shown in FIG. 12, electropermanent magnets 1210,1220 clamp electropermanent linear actuator 1230 to iron plate 1240.

Operation of the inchworm motor: If both clamp magnets are turned on,the inchworm holds itself in a fixed position on the iron plate. To movein one direction, one clamp is turned off, the linear actuator iscompressed, the clamp is turned on, the other clamp is turned off, thelinear actuator is expanded, and the other clamp is turned back on. Theinchworm will now have moved to a new position.

It will be clear to a person skilled in the art of the invention that itis possible to combine multiple inchworm motors facing in differentdirections in order to achieve an inchworm motor capable of translationand rotation in multiple directions. It will also be clear to a personskilled in the art that the clamp magnets may have curved faces to allowthe inchworm motor to move on the inside or outside faces of a cylinder,sphere, or other curved shape. It will further be clear to one of skillin the art that these embodiments are exemplary only, and that many ofthe variations discussed in conjunction with the motor of FIGS. 1A-B and2A-B would also be suitable for use in an electropermanent inchwormmotor according to the present invention.

While preferred embodiments of the invention are disclosed, many otherimplementations will occur to one of ordinary skill in the art and areall within the scope of the invention. Each of the various embodimentsdescribed above may be combined with other described embodiments inorder to provide multiple features. Furthermore, while the foregoingdescribes a number of separate embodiments of the apparatus and methodof the present invention, what has been described herein is merelyillustrative of the application of the principles of the presentinvention. Other arrangements, methods, modifications, and substitutionsby one of ordinary skill in the art are therefore also considered to bewithin the scope of the present invention, which is not to be limitedexcept by the claims that follow.

1. An electropermanent magnet-based motor, comprising: a stator, thestator comprising: at least one electropermanent magnet; and at leastone coil located around the electropermanent magnet, the coil beingconfigured to pass current pulses that affect the magnetization of theelectropermanent magnet; and a rotor, the rotor being movable withrespect to the stator in response to changes in the magnetization of theat least one electropermanent magnet.
 2. The motor of claim 1, thestator further comprising: a centrally-located stator core; and aplurality of stator arms radiating outward from the stator core, atleast one electropermanent magnet being integral to each stator arm anda coil being located around the at least one electropermanent magnet oneach stator arm, the stator being located within the rotor in such aconfiguration that the rotor may rotate around the stator arms.
 3. Themotor of claim 2, wherein the stator core and stator arms are formedfrom a single piece of material.
 4. The motor of claim 1, wherein the atleast one electropermanent magnet comprises twoindependently-controllable magnets made of materials having differentcoercivities.
 5. The motor of claim 2, wherein the at least oneelectropermanent magnet comprises two independently-controllable magnetsmade of materials having different coercivities.
 6. The motor of claim5, wherein one of the two magnets is an Alnico alloy magnet and theother is a neodymium alloy magnet.
 7. The motor of claim 1, furthercomprising motor drive circuitry.
 8. The motor of claim 2, furthercomprising motor drive circuitry.
 9. The motor of claim 2, the end ofeach stator arm further comprising integrated stator gear teeth androtor interior further comprising integrated rotor gear teeth, thestator gear teeth meshing with the rotor gear teeth.
 10. The motor ofclaim 2, further comprising: at least one coaxial gear wheel or frictionroller, the gear wheel or friction roller comprising wheel teeth andbeing located above and below the plane occupied by the stator; and therotor further comprising a ring sandwiched between two rotor gearshaving rotor teeth designed to mate with the wheel teeth of the statorgear wheel or friction roller.
 11. The motor of claim 1, furthercomprising a high-friction coating on at least one of the rotor orstator surfaces.
 12. The motor of claim 1, the stator furthercomprising: anteriorly-located stator core, the stator being exterior tothe rotor; and a plurality of stator arms radiating inward from thestator core at least one electropermanent magnet being integral to eachstator arm and a coil being located around the at least oneelectropermanent magnet on each stator arm, the rotor beingcentrally-located within the stator in such a configuration that therotor may rotate within the stator arms, the rotor further beingconfigured to rotate about its axis.
 13. The motor of claim 12, whereinthe stator core and stator arms are formed from a single piece ofmaterial.
 14. The motor of claim 12, wherein the at least oneelectropermanent magnet comprises two independently-controllable magnetsmade of materials having different coercivities.
 15. The motor of claim14, wherein one of the two magnets is an Alnico alloy magnet and theother is an neodymium alloy magnet.
 16. The motor of claim 12, furthercomprising motor drive circuitry.
 17. The motor of claim 12, the end ofeach stator arm further comprising integrated stator gear teeth and therotor exterior further comprising integrated rotor gear teeth, thestator gear teeth meshing with the rotor gear teeth.
 18. The motor ofclaim 12, further comprising a shaft located at the rotor axis.